The present invention relates to a variable-valve-actuation (VVA) apparatus for internal combustion engines, which can vary the lift amount of engine valves such as intake valve and exhaust valve in accordance with the engine operating conditions.
As is well known, the intake and exhaust valves are opened and closed by a cam shaped, e.g. like a raindrop and fixed to a camshaft rotated in synchronism with a crankshaft. The cam has an outer periphery or profile with which a base circle face for zero-lift period, a ramp face for ramp or cushioning period connected to the base circle face, and a lift face or event portion for lift period connected to the ramp face are formed continuously.
The ramp period includes an up-lift period at rising of the valve lift and a down-lift period at termination of the valve lift, during which the lift rising velocity and the lift lowering velocity are restrained to small values, respectively. Such small lift velocity allows cushioning of an excessive impact stress applied on the intake valve or the exhaust valve.
Recently, there are provided internal combustion engines which comprise a VVA apparatus including an alteration mechanism for variably controlling the valve lift amount in accordance with the engine operating conditions.
The VVA apparatus comprises a low-velocity cam, a medium-velocity cam, and a high-velocity cam disposed adjacent to each other and fixed to a camshaft rotated in synchronism with a crankshaft. The cams having different profiles are selectively switched in accordance with the engine operating conditions to change the height of the lift face for enhancement of the engine performance.
For the ramp period, the profile of each cam is established to provide cushioning. However, a specific influence on the engine performance due to the ramp period is not considered to a sufficient degree.
Specifically, during the ramp period, the low-velocity cam for use in the low-rotation low-load range including idle running produces impact noise such as lift starting noise at opening of the engine valve or seating noise at closing thereof, which is heard relatively loudly since drive noise of the whole engine is small in this operating range.
Moreover, the high-velocity cam for use in the high-rotation range produces; loud noise due to unusual behavior of the engine valve such as bounce or jump, which cannot be restrained since the valve-lift starting velocity and the engine-valve seating velocity are very high in this operating range.
Further, in the medium-rotation high-load range having less possibility of occurrence of singular noise to be produced in the above two ranges, the engine valves suffer substantially advanced opening timing and substantially delayed closing timing, leading to deterioration of the intake and exhaust efficiency.
It is, therefore, an object of the present invention to provide a VVA apparatus for internal combustion engines, which contributes to a reduction in impact noise in the low-rotation low-load range and prevention of unusual behavior of the engine valves in the high-rotation range with enhanced intake and exhaust efficiency in the medium-rotation and high-load range, etc.
The present invention provides generally a variable-valve-actuation (VVA) apparatus for an internal combustion engine, comprising: a valve; and a mechanism which variably lift characteristics of the valve in accordance with operating conditions of the engine, wherein the lift characteristics include a ramp period which is shorter in a range of medium lift amount than in a range of small lift amount and a range of large lift amount.